1. Field to Which the Invention Relates
The invention relates to a non-recursive digital filter for generating digital-coded output samples which occur at a given output sampling frequency (f.sub.u) and which are related in a predetermined way to a sequence of digital-coded input samples occurring at an input sampling frequency (f.sub.i), the input sampling frequency being an integral multiple (r) of the output sampling frequency; comprising a first storage device arranged to store a given number of successive input samples; an input circuit for supplying successive input samples to said first storage device; a multiplier; means for supplying said input samples stored in said first storage device to said multiplier; means for supplying weight factors which correspond to the relationship between the input and output samples to said multiplier, said multiplier forming products of a given number (N) of said input samples and a number (N) of said weight factors; an adder coupled to said multiplier and arranged to supply, within one output sampling period (1/f.sub.u), an output sample, which is given by the mathematical sum of the product formed within that period.
2. Description of Prior Art
Non-recursive digital filters of the type described above are known. For example, in a known filter r sets are formed for the calculation of an output sample in which each set consists of the sum of N products obtained by multiplying N input samples by N weight factors. Of said r sets one is read out to supply an output sample. For the calculation of one output sample rN additions and rN multiplications are performed in this filter in a period T.sub.u (=1/f.sub.u) while due to the reduction of the sampling frequency, only N sums and products need to be determined.
In the Dutch Patent Application No. 7400761 a digital filter is described in which only N multiplications and N additions are performed in a period T.sub.u for calculating an output sample. Particularly in the embodiment of FIG. 7 of that application a non-recursive digital filter is used as first filter section comprising a time demultiplexer having r outputs, a buffer memory being connected to each output. The r buffer memories are read out at a frequency f.sub.u and the stored input samples are supplied to r multipliers for each forming a set of N/r products which supply an output sample after having been added up. The filter described here can be used to advantage when N/r is an integer; however, part of this advantage is lost when N/r is not an integer because the group of N weight factors must be completed to a multiple of r, which increases the number of multiplications and additions. SHORT DESCRIPTION OF THE INVENTION
The invention has for its object to provide a universally applicable non-recursive digital filter of the type described above, in which the output sampling frequency is a factor r lower than the input sampling frequency and in which also only N products of input samples and weight factors need to be determined for calculating an output sample.
The filter according to the invention is therefore characterized in that said input circuit comprises a second storage device arranged to store a group of input samples, which group is formed by a given number of successive input samples, and means for writing said group of input samples into said first storage device in the rhythm of occurrence of the output samples.